A nanocrystal, also known as a nanocrystalline, is a material which has a size of a few nanometers, usually in a range of 1-20 nanometers, and has a crystal structure. In general, a nanocrystal refers to a structure having a spherical or nearly spherical inner crystal core with one layer or a plurality of layers of shells wrapped thereon. Of course, a nanocrystal also includes a structure without a shell. A luminous nanocrystal or fluorescent nanocrystal refers to a crystal that has a nanocrystal structure, and emits light when being excitated by an appropriate light source or voltage. Luminous characteristic of the fluorescent nanocrystal may be controlled through the composition, size and shell surface structure of the nanocrystal. Therefore, relative to organic materials, a fluorescent nanocrystal can provide excellent color purity, color diversity, photons and thermal stability. The luminous core may be a sphere, a strip, a rectangle, or even other polyhedral shapes having a total volume of not larger than 20 nm×20 nm×20 nm. The luminous stability of the nanocrystal may be improved through growth of a shell or a plurality of shells on the nanocrystal core. The fluorescent nanocrystal core may be composed of a metal, a metal oxide and semiconductor materials including compounds of Groups II-IV and III-V. The composition of the fluorescent nanocrystal may be changed by doping with one or more transition metal cations, so as to change the luminous wavelength and other luminous properties of the nanocrystal.
The fluorescent nanocrystal has been widely researched over the past 20 years, due to its special properties, such as optical properties that can be controlled through adjustment of size, high quantum efficiency, relatively narrow peak width at half height, and photodegradation resistance. Compared with organic dyes, the fluorescent nanocrystal, as a new-generation luminous material, has wider usages in many applications, such as luminous display devices, photovoltaic devices and biomarkers.
In general, the fluorescent nanocrystal may be synthesized by pyrolyzing a metal complex in the presence of a hydrophobic solvent at a temperature of 200-350° C. The fluorescent nanocrystal may also be prepared by using water, ethanediol and other hydrophilic solvent as reaction solvents at room temperature or elevated temperature. In order to achieve uniform reaction, reactants are usually dissolved in a solvent at room temperature or elevated temperature. Insoluble inorganic substances or reactants in a gaseous state may also be used to prepare semiconductor nanocrystals. However, nonuniform reaction and chemical non-equilibrium will generally cause poor repeatability and quality of nanocrystal synthesis. Reactants in a gaseous state, such as hydrogen sulfide, hydrogen phosphide and hydrogen arsenide, may also be used to prepare nanocrystals due to their high reactivity. When the nanocrystal is prepared by using the gas according to the prior art, the prepared gas precursor is usually directly supplied to a metal precursor. In this manner, the nanocrystal may also be obtained.
However, the synthesis of nanocrystal by using the in-situ precursor in a gaseous state has the following problems. The reaction is uncontrollable and difficult to be repeated. This is because the use level of a precursor in a gaseous state cannot be accurately controlled, which may lead to non-repeatable reaction, and may also lead to heterogeneous nucleation and nanocrystal growth. In addition, excessive gas not being reacted needs to be treated by an additional device or a cleaning process. An additional gas formation device and a drying system are required, since oxygen/water will affect the nanocrystal quality. Moreover, the above synthesis of nanocrystal is highly toxic and difficult to be treated. This is because hydrogen phosphide, hydrogen arsenide and like gases, which are highly toxic and need strict operation procedures, can be safely treated by well-trained specialized persons only.